Dr. S. H. Burungale

1. GAS CHROMATOGRAPHY
Dr. S. H. Burungale
Head Department of Chemistry
Yashwantrao Chavan College of Science,
Karad-415124

2. GAS CHROMATOGRAPHY(GC)
Chromatography is a physical method of
separation and analysis. Contrast to chemical
methods, this method is very quick, can be used
with very small quantity of sample and the analysis
can be done with high accuracy and precision.

3. In chromatography, the components of a
mixture are distributed between two phases : (1)
Stationary phase having large surface area (a
porous silica or liquid coated on inert porous solid)
and (2) mobile phase (a gas or liquid) which
moves continuously in contact with stationary
phase.

4. When the mobile phase is gas, it is termed
as gas chromatography and depending on the
stationary phase used, it is either Gas-solid
chromatography (GSC) or Gas-liquid
chromatography (GLC).

5. In, GC elution method of development is
used. Here inert mobile phase gas, is kept flowing
continuously in contact with stationary phase in
column. The components of sample either as
mixture or partially separated are transported from
one end of the column to the other end of column.

6. In GC, the stationary phase either finely
divided solid or liquid coated on inert solid is tightly
packed in a column with narrow bore. It is 3 to 5
meter long. Column is kept in an oven maintained
at constant temperature. A carrier gas which is
relatively inert like N2, Ar, H2 or He is introduced
through one end and is kept constantly flowing in
the column.
Principle of GC Separation

7. A sample is introduced at a point which is
few centimeters away from entrance point of
carrier gas. If liquid mixture of sample is introduced
it is instantly vaporised in a heated port and
inserted as a sharp plug.

8. When the components of a mixture carried by
carrier gas comes in contact with solid stationary
phase, it gets adsorbed. Adsorption occurs
according to the Freundlich adsorption isotherm;
x/m = KC1/n or
The Langmuir adsorption isotherm
x/m =
C
K
C
K
2
1
1

9. Where x is amount of solute getting
adsorbed on m gm of stationary phase, C is
concentration of solute in gaseous state and all K
are constants. n is integer.
If stationary phase is liquid the solute gets
dissolved and Henry's law is followed
x/m = KC

10. Both the phenomenon are selective. K
values are different for different solutes on the
same sorbent and hence different amount of solute
is going to stationary phase. An equilibrium is
established between the solute on stationary
phase and solute in mobile phase.

11. As mobile phase is constantly flowing, the
amount of solute not adsorbed is swept away with
mobile phase and to maintain the equilibrium and
K value, out of the swept amount of solute, some
will be adsorbed on the next point again to
maintain K value. This adsorption and desorption
keeps on going successively at every point in the
column.

12. Now components having different K value on
that stationary phase, will be retained on column
differently and hence each solute will travel with
different speed. Each solute follows a Guassian
distribution and travels with the shape of a peak.
As the length of column is quite large, each solute
will come out of the column at different time.

13. If a suitable detector is kept at the other end
of column, then as soon as solute enters it, a
signal will be obtained which is fed to recorder. On
recorder the peak for each solute appears at
different time, and the graph showing multitude of
peaks called Chromatogram is obtained.

14. 0X
0X
0X
0X
oX
Carrier
gas
Detector
Sample
(Binary mixture)
Sharp plug
Of sample
OX
OX
OX
OX
OOXX
OOXX
OOXX
OOXX
OO
OO
OO
OO
OO
OO
OO
OO
XX
XX
XX
XX
(i) (ii) (iii) (iv)
XX
XX
XX
XX
Progress of separation

15. Instrument used in GC

16. Mobile phase used in GC
Depending on the mixture to be separated
and detector used, an inert gas like N2, Argon, H2
or He is used as carrier gas. Generally with FID,
N2 or Argon is used and with TCD H2 or He is
used.

17. Sample introduction system
A liquid sample in microlitre quantity is
introduced using hypodermic syringe, while
gaseous sample is introduced with gas tight
syringe or using gas sampling valve.

18. In GC separation occurs in gaseous state
and hence the liquid sample introduced has to be
vaporised instantly. Injection port is therefore kept
heated at a temperature above the highest boiling
point of a component of a mixture.

19. Column
It is considered to be heart of GC, where
separation occurs. Columns are of 3 types.
(1) Packed columns
(2) Wall coated open tubular columns (WCOT)
(3) Support coated open tubular columns (SCOT)

20. Packed columns are of 3 to 5 m length with
internal diameter of 1.5 to 6 mm. Here stationary
phase solid or inert solid on which liquid stationary
phase is coated is packed as micron sized
particles.
(1) Packed columns

21. WCOT is called a capillary column which is
having length of 10-100 m. with internal diameter
0.2 to 0.8 mm. Here liquid stationary phase is
directly coated on wall. Due to extra length, any
complex mixture can be separated with good
resolution. As there is no packing inside, the
carrier gas does not suffer any resistance and
separation is very fast. Its drawback is that, it
cannot accommodate more sample.
(2) Wall coated open tubular columns

22. SCOT has length 10-25 m. and internal
diameter 0.5 mm. Here liquid stationary phase is
coated on micron size layer of inert solid. With
similar advantages of WCOT, more sample can be
loaded on this column.
(3) Support coated open tubular columns

23. GSC has solid stationary phase. Solid
stationary phase used are activated carbon, silica
gel, alumina or molecular sieves. GSC has very
limited applications. It is used to separate
permanent gases and low molecular weight
hydrocarbons. GSC has drawback that due to non
linear adsorption isotherms, peaks show trailing.

24. Use of porous polymer as stationary phase is
important. Divinyl benzene and styrene are
copolymerised under controlled condition to give
porous beads of polymer. Many times a polar liquid
monomer is incorporated to give porous polymer
with definite polarity. Such polymers are
manufactured and available with trade name of
“Porapaks”. They are available with different
polarity and termed as Porapak-P, Q, R,S,N and T.

25. Some separations using porous polymers
are unique. (1) Separation of a mixture of reactive
gases like Cl2, HCl, HCN etc. (2) Separation of a
mixture of formaldehyde, water and methanol.
(3) Mixture of ethylene, ethane, acetylene.
Recently polymer lined open tubular columns
(PLOT) which is 30 m long and ID 0.53 mm. is
introduced. It can separate a mixture of air,
methane, CO2, ethylene, ethane.

26. GLC is more used than GSC due to certain
reasons.
(1) It can be used to separate high molecular
weight liquids.
(2) It is faster than GSC.
(3) The peaks do not show trailing and they are
symmetrical peaks.
(4) Wide choice of liquid stationary phase.
Gas Liquid Chromatography (GLC)

27. The liquid stationary phase is coated on inert
solid. Such inert solid particles are uniformly
shaped, sized and have size less than 10 µm.
Most widely used inert solids are
diatomaceous earths. They are skeletal
remains of the unicellular algae known as
diatoms.
(i) white diatomaceous earths
(ii) pink diatomaceous earths.

28. They differ in method of preparation. They
are manufactured by John-Manville Corp. under
trade name of Chromosorb-W, Chromosorb-P.
Besides these, teflon, glass beads,etc. are used
but not have become popular.
Liquid stationary phase used should be non
volatile at operating temp. of column, should not
be viscous, and should be suitable for the
components to be separated.

29. Most widely used liquid stationary phases
with their temperature limits and applications are
given below.
They are classified as;
(1) Nonpolar
(2) Polar
(3) Intermediate polar etc.

30. (a) Polar liquid stationary phases
(1) Dimethyl sulpholane
(2) Oxydipropionitrile
(3) Carbowax-M
(4) Versamid (Polyamide resin)
(b) Non Polar
(1) Hexadecane
(2) Squalene
(3) Methylsilicon gum
(4) SE-30
(c) Intermediate Polar
(1) SE-52
(2) Didecylphthalate
(3) DC-550
(4) Benzylbiphenyl

31. Generally to separate polar solutes, polar
liquids are used and to separate non-polar solutes,
non-polar liquids are used as stationary phase.
Liquid stationary phase is dissolved in
volatile solvents, mixed with inert solid support in
definite proportion and then volatile solvent
removed by evaporation, when liquid stationary
phase gets coated on inert solid support.

32. It is then filled in a column to get a packed
column. Generally ready made columns are
purchased from skilled manufacturer.
Columns are kept in oven which is heated
electrically. Its temperature is controlled
electronically.

33. Most widely used detectors are;
(1) Thermal conductivity detector (TCD)
(2) Flame ionisation detector (FID)
(3) Thermionic detector (TID)
(4) Electron capture detector (ECD)
(5) Flame photometric detector (FPD) and
(6) Sulphur chemiluminiscence detector (SCD)
The first two detectors being very common will
not be discussed. Only their comparison is given.
Detectors

34. TCD FID
It responds to all compounds. It is
universal detector.
It responds only to organic compounds
and does not respond to NH3, H2O,
H2S, nitrogen oxides, sulphur oxides
etc.
It is less sensitive than FID. It is more sensitive than TCD.
Its sensitivity depends on flow rate of
carrier gas.
Its sensitivity does not depend upon
flow rate of carrier gas.
It is non destructive detector and hence
suitable with preparative GC.
It is destructive detector and can not be
used with preparative GC.
Here carrier gas used are He or H2. Here N2 or Ar used as carrier gas.

35. It is modified FID. It is selective for
phosphorous and nitrogen containing compounds.
Compared to FID, this detector is 500 times more
sensitive for P compounds and 50 times more
sensitive to N compounds. It is useful for detection
and measurement of P containing pesticides.
Thermionic Detector (TID)

36. Hydrogen
inlet
Igniter
Chimney
Exhaust
Collector electrode
Polarizing electrode
Air or oxygen
inlet
Column effluent

37. It is similar to FID, but in addition it contains an
electrically heated rubidium silicate bead near
collector electrode, which is maintained at about
180 V with respect to collector electrode. The hot
gases then flow around the bead. The heated bead
forms a plasma (gaseous conducting mixture
containing ions) having temperature 6000 to 8000C.
In this plasma, N & P compounds form large no. of
ions, to give large current, and high sensitivity.

38. This is a selective detector which is used for
phosphorous and sulphur containing compounds.
Its construction differs from FID. Like FID it
does not contain the collector electrode around the
flame and does not measure the amount of
ionisation. Instead it measures the radiation
emitted by the flame by the sample components.
For this it contains a photomultiplier tube with
suitable filters. It is shown in the diagram.
Flame photometric detector (FPD)

39. When the eluent is passed in low
temperature hydrogen-air flame, phosphorous
converts to HPO species which emits radiation
centered about 510 and 526 nm.
Sulphur simultaneously is converted to S2
species which emits radiation centered at 394 nm.
Desired wavelength is isolated by using a filter
between the flame and photomultiplier tube.

40. It can also be used to detect halogens,
nitrogen compounds and organometallic
compounds containing metal chromium, selenium
and germanium.
For sulphur compounds, sulphur
chemiluminescence detector (To be described
later) provides greater working range and more
sensitivity than FPD.

41. H2 gas Column
effluent
Air
PM
tube
Filter

42. It is the recent addition, to the family of
detectors used in GC.
It is based upon the reaction between certain
sulphur compounds and ozone. The reaction
produces luminescence and the intensity of this
chemiluminescence is proportional to the
concentration of sulphur. The detector is
particularly useful for the determination of
pollutants such as mercaptans.
Sulphur Chemiluminiscence Detector (SCD)

43. In SCD, the eluent is mixed with hydrogen
and air and is burned as in FID. The resulting
gases are then mixed with ozone and the intensity
of resulting luminescence is measured.
For sulphur compounds, it offers very high
sensitivity and more linear working range.

44. Temperature of column is very important in
separation, as the partition coefficient of the solute
between the two phases much depends on
temperature. As column temperature increases,
the sample spends more time in mobile phase.
This results in decreasing retention time. (Time for
which solute remains in the column). Opposite to
that at lower column temperature, the solute is
retained more on stationary phase and retention
time increases.
Temperature Programming

45. If a complex mixture is to be analysed in which
solute components have a large difference in boiling
point, then the separation at constant column
temperature (isothermal mode) can create
difficulties. At low column temperature, the lower
boiling solutes are well resolved and eluted in
reasonable time but the higher boiling solutes take
long time to elute and few may not come out of the
column. To decrease the retention time of higher
boiling solutes, if column temperature is taken high
then lower boiling solutes are eluted so quickly that
they are not properly resolved.

46. This problem can be overcome by using
temperature programming. In this technique initially
the column is kept at low temperature, and then its
temperature is raised in a programmed manner at
desired heating rate like 50C/min, 100C/min etc.
When the column is at low temperature, the lower
boiling solutes eluted effectively, while the higher
boiling solutes remain condensed on the column.
As the temperature is gradually raised, they start
evaporating from column and elute in the order of
their boiling points.

47. To do temperature programming, initial
temperature, desired heating rate and final
temperature are set on electronic controls. Many
gas chromatographs have facility for this.

48. Though very efficient separation and
analytical technique, GLC has limitation that it
cannot be used for separation of non volatile liquids
and solids. Liquids having maximum boiling point
2500C can be separated with GLC. Nonvolatile
liquids or solids, if desired to be separated by GC,
they have to be converted to their volatile
derivatives. This is called derivatisation.
Derivatisation

49. The reaction used to prepare volatile
derivatives of non-volatile compounds, must be
simple, fast and quantitative even at very low
concentrations encountered with GLC. A few
reagents used to prepare volatile derivatives for
different class of compounds have been listed
below.

50. No. Derivative Reagent Applications
1 Methyl esters Diazomethane, BF3 methanol Acids
2 Silyl ethers N, o-Bis (trimethylsilyl) acetamide,
Trimethylsilylimidazole
Alcohols, Acids,
Amides, amines.
3 Fluoroacetylaceronates
(Volatile complexes)
Trifluoroacetylacetone Metals
4 Fluoroacetates Trifluoroacetic anhydride Alcohols, Amines,
aminoacids.

51. Head Space Gas Chromatography
Due to very sting quality control parameters
needed in finished products, to be exported, it
becomes essential to find percentage of volatiles
in finished products (solid or liquid) e.g. impurity of
solvent or PCB, in solid or liquid has to be
determined. For this HSGC is used.

52. The measured quantity of product is taken in
an injection vial and sealed. It is kept in a heated
port where volatiles are evaporated and collected
in head space. A syringe needle is inserted in
rubber cap of vial and vapours collected from head
space, are sucked in syringe. It is then removed
and gases from syringe are introduced in injection
port of GC and analysed. All operations are
performed by a robot.

53. GC-interfaced with Mass Spectrometer
Though GC is very useful in quantitative
analysis, qualitative analysis using GC is almost
impossible.
In research, when a plant extract is analysed
by GC, to identify each organic compound
obtained from it, GC is interfaced with MS.

54. Each component separated is undergoing
fragmentation pattern and mass spectra obtained,
the unknown can be identified. Here again
computer acquisition data makes the things very
simple.

55. Modern Elemental Analyser
Modern elemental analyser to determine
percent of C, H and N in organic compound,
simultaneously, principle of GC is used.
A tin crucible containing exact weighed
quantity of organic compound, is dropped in a
furnace heated at 9500C. A flow of oxygen is
introduced. C, H & N are converted to CO2, H2O &
NOx respectively.

56. The mixture passed over copper gauze
when, NOx are converted to N2. This mixture of
CO2,H2O and N2 is then led to a chromatographic
column filled with porapak, where they are
separated and detected by TCD. From percent of
CO2, H2O and N2, percent of C, H and N can be
determined. It has following advantages.
• Hardly 2-5 mg substance needed.
• Percentage of element determined within
5 minute with excellent accuracy.